NASA-led study unravels mystery of 'smooth terrains' on icy moons circling Jupiter and Saturn

For the study, scientists calculated the size of steep ridges formed by tectonic faults on moons, similar to those found on Earth. After obtaining the measurements, the team applied them to computer-based seismic models to estimate the strength of past moonquakes.

Interestingly, the findings revealed that previous quakes were so powerful that they triggered landslides on the moons. The debris was carried downslope, where it spread out and eventually smoothed the surface. 

“We found the surface shaking from moonquakes would be enough to cause surface material to rush downhill in landslides. We’ve estimated the size of moonquakes and how big the landslides could be. This helps us understand how landslides might be shaping moon surfaces over time,” said Mackenzie Mills, lead author of this study from the University of Arizona in Tucson, in a statement.

One of the computer-modeled observations of tectonic activity and quakes on Saturn's moon Enceladus revealed: “Because of that moon’s small gravity, quakes on tiny Enceladus could be large enough to fling icy debris right off the surface and into space like a wet dog shaking itself off,” added Robert Pappalardo, project scientist of Europa Clipper at JPL. 

This is the first time that quakes, which caused landslides, have been linked to the cause of unusually smooth terrain. Previously, scientists hypothesized that the smooth surface was caused by liquid erupting from icy volcanoes. However, the icy moons are so cold that liquid flow would be nearly impossible to spread on the moon's surface. 

The team intends to learn a lot more about the icy moons found in the far reaches of our solar system. The upcoming NASA's Europa Clipper mission, which will conduct close flybys of Jupiter's moon Europa, will be able to further expand this study. However, that would not happen until the end of this decade. After arriving at Jupiter in 2030, this mission will collect imagery as well as other scientific data that could help to solve some of Europa's mysteries, such as determining the habitability of the ocean beneath the moon's ice shell.

Study Abstract:

Intense tectonism is evident on many outer solar system satellites with some surface regions exhibiting ridge-and-trough structures which have characteristics suggestive of normal faulting. In some cases, topographic lows between subparallel ridges are sites of smooth material displaying few craters. We consider whether such smooth material can be generated by mass wasting triggered from local seismic shaking. We hypothesize that debris would flow from topographic highs into lows, initially mobilized by moonquake-induced seismic shaking during formation of local tectonic ridges, covering and infilling older terrain. We analyze the feasibility of seismicity to trigger mass movements by measuring fault scarp dimensions to estimate quake moment magnitudes. The inferred magnitude range is 4.0–7.9, and we use numerical modeling to estimate seismic accelerations resulting from such quakes. This modeled magnitude range implies seismic accelerations that can exceed satellite gravitational accelerations, particularly near quake epicenters. Thus, seismic events could feasibly cause mass wasting of material to form some fine-scale smooth surfaces observed on at least three icy satellites: Ganymede, Europa, and Enceladus.